An organic EL device to be used for various displays, for example, large-size displays such as a television and small-size displays such as a mobile phone, a personal computer, and a smartphone is generally manufactured by forming thin film transistors (hereinafter referred to as “TFTs”) on a glass substrate serving as a supporting base, successively forming an electrode, a light-emitting layer, and an electrode on the glass substrate having the TFTs formed thereon, and finally hermetically sealing the resultant with a separate glass substrate, multi-layered thin film, or the like. As structures of the organic EL device, there are given a bottom-emission structure in which light is extracted from the side of a glass substrate serving as a supporting base and a top-emission structure in which light is extracted from the side opposite to a glass substrate serving as a supporting base, and those structures are used properly depending on the applications. As another structure of the organic EL device, there may be adopted a structure that allows outside light to directly pass therethrough, and hence there has also been proposed a transparent structure in which electronic elements such as TFTs are visible from the outside. All of the above-mentioned structures can be realized by selecting transparent electrodes and substrate materials.
In addition, the organic EL device can be made thin, lightweight, and flexible by substituting a resin for the related-art glass substrate as the supporting base of the organic EL device. As a result the applications of the organic EL device can further expanded. However, the resin is generally inferior to glass in dimension stability, transparency, heat resistance, moisture resistance, gas barrier property, and the like, and hence various studies of the resin have been conducted.
For example, JP 2008-231327 A (Patent Literature 1) relates to the invention of a polyimide and a precursor thereof useful as a plastic substrate for a flexible display and has reported that polyimides obtained by subjecting tetracarboxylic acids containing an alicyclic structure such as cyclohexylphenyl tetracarboxylic acid to reactions with various diamines are excellent in transparency. However there is a problem in that the polyimides obtained in the foregoing have a glass transition temperature of up to 337° C. according to Examples (Table 1), and hence cannot withstand a heat treatment temperature generally reaching about 400° C. during the annealing step of TFTs. In addition, all of the obtained polyimides have a coefficient of thermal expansion (CTE) of from about 50 ppm/K to about 60 ppm/K. Therefore, in the case where a gas barrier layer is formed so as to impart gas barrier property to the polyimide as in Patent Literature 2 described later, peeling and cracks occur at an interface between the gas barrier layer and the polyimide, with the result that it is difficult to obtain an organic EL device excellent in shape stability.
Further, JP 2011-238355 A (Patent Literature 2) relates to the invention of a gas barrier film that is excellent in gas barrier property and heat resistance, is flexible, and can be used as a base for an organic EL device or the like, and discloses the following: a flexible film made of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polyvinyl chloride (PVC), polyimide, or the like is used as a base; a stress relaxation layer and a gas barrier layer (inorganic barrier layer) including a compound containing at least silicon and oxygen are formed on one surface side of the flexible film so as to prevent the permeation of water vapor and air; and the coefficient of thermal expansion of the stress relaxation layer is set within a range of from 0.5 ppm/K to 20 ppm/K so as to prevent the occurrence of peeling and cracks caused by differences in thermophysical properties (coefficient of thermal expansion, thermal shrinkage rate) between the resin base and the inorganic barrier layer. However, there is a problem in that the flexible film made of PET, PEN, PC, PVC, or the like given as the supporting base in the foregoing does not have sufficient heat resistance and hence cannot withstand the heat treatment temperature generally reaching about 400° C. during the annealing step of TFTs. Further, polyimide (gas barrier film 2-3) used in Comparative Example has a transmittance lower than that of glass due to a yellowish brown color thereof and hence is not preferred as a resin to be substituted for glass.
Further, JP 2007-46054 A (Patent Literature 3) relates to the invention of a low tinted polyimide resin composition useful for glass type applications in the field of electronic displays, and discloses that a polyimide film containing a perfluoro-imide moiety has a low coefficient of thermal expansion and a high glass transition temperature and is excellent in transparency. However, the transmittance in a visible light region of most of the polyimide films actually obtained in Examples has not reached 80%, and the glass transition temperature thereof has not reached 400° C. Thus, a polyimide film that concurrently satisfies low thermal expandability, transparency, and heat resistance has not been obtained.
Similarly, JP 2-251564 A (Patent Literature 4) discloses that a fluorine-containing polyimide composition containing a fluorinated alkyl group introduced into an acid anhydride and a diamine has a low dielectric constant, a low water absorption coefficient, and low thermal expandability and is applicable to materials for printed boards and optical waveguides. However, Patent Literature 4 does not describe the transmittance in a visible light region of a polyimide film. Further, Patent Literature 4 does not describe means for solving the retardation problem caused when a transparent polyimide film having a low coefficient of thermal expansion is applied to a supporting base of a display device.
Besides the above, attempts have been made to reduce the weight of a device by rising a flexible resin for a supporting base. For example, Non Patent Literatures 1 and 2 listed below have proposed organic EL devices in which polyimide having high transparency is applied to the supporting base. However, as described above, it cannot be said that the difference in coefficient of thermal expansion between the polyimide films described in those literatures and the gas barrier layer of an inorganic compound formed so as to compensate for gas barrier property is sufficiently small.